Illegal drugs such as heroin and fentanyl are fueling the opioid epidemic despite progress in getting doctors to prescribe fewer painkillers.

Federal officials and lawmakers say more must be done to stem a growing tide of overdose deaths from illicit opioids such as heroin and the synthetic opioid fentanyl, which is even more potent and cheaper than heroin.

The emergence of fentanyl especially has caught the attention of Congress, with lawmakers trying to clamp down on the sources of heroin and fentanyl.

A few weeks ago, Sens. Rob Portman, R-Ohio, and Amy Klobuchar, D-Minn., asked the Drug Enforcement Agency to provide additional information to law enforcement officers on how to handle fentanyl overdoses.

"Incidental exposure to synthetic opioids is difficult to detect and potentially fatal, and we are deeply concerned by recent reports of officers who have suffered accidental overdoses after coming into contact with these substances while at work," the senators said in a letter to the DEA.

Portman has been a major fighter in combating opioid abuse, spearheading the Comprehensive Addiction and Recovery Act that was signed into law by former President Barack Obama last year.

He and Klobuchar cosponsored legislation this year to help the U.S. Customs and Border Protection crack down on fentanyl shipments from overseas. The legislation, called the Synthetic Trafficking and Overdose Prevention Act, has not advanced in the chamber.

Congress is focusing now on how much money needs to be devoted to fighting opioid abuse. The Republican health bill being considered in the Senate would add $45 billion to fight the epidemic, which is more than $2 billion in the House version. But experts have questioned whether that would be enough.

But there was some good news recently on the fight to curb opioid deaths.

For the first time, there was a decline in the number of painkillers prescribed in 2015, according to a recent report from the Centers for Disease Control and Prevention, although the amount of prescriptions was still extremely high.

In 2015, U.S. doctors prescribed three times the amount of painkillers such as oxycodone and hydrocodone as in 1999, according to the CDC.

"We are starting to see progress on the prescribing side of the issue, but there is still an enormous way to go," said Dr. Deborah Dowell, senior medical adviser at the CDC's Division of Unintentional Injury Prevention.

Turning the tide on overprescribing has been a major priority of the federal government for years. The CDC published guidelines in 2009 and 2010 warning doctors about the high dosages of opioids, and it appears some prescribers listened, Dowell said.

The CDC also is boosting training for doctors to educate them on the proper ways to prescribe opioids, which often were given out too liberally.

However, the number of deaths from overdoses remains high despite more knowledge about the effect of opioids.

In 2015, 21,088 people died from a prescription opioid painkiller overdose, a 35 percent increase from the 16,651 who died in 2010.

But that isn't the biggest increase over the period. The number of people who died from an overdose of heroin or fentanyl soared to 19,884 in 2015 from 5,998 in 2010, according to the CDC.

"One of the biggest problems is that people who started [on] prescription opioids go on to [take] illicit[ly] manufactured heroin or illicitly manufactured fentanyl, which is more potent and unsafe than heroin," Dowell said.

Fentanyl has exploded in popularity among drug addicts. From 2015 to 2016, the DEA seized double the amount of drugs containing the synthetic opioid.

Awareness about the dangers of fentanyl have increased over the past year, especially after music icon Prince died from an overdose last year.

Most people who use fentanyl or heroin do so by moving on from prescription painkillers, emphasizing the importance of more training for prescribers, Dowell said.

"There is a lot of people who have already been exposed to opioids," she said. "We need to help those people and get them effective treatment for opioid use disorder, and at the same time, we need to prevent more people from being unnecessarily prescribed in the first place."

The CDC is putting together educational materials to try to get prescribers to dole out fewer painkillers. The agency also is pushing alternatives for managing pain besides drugs, such as physical therapy.

The CDC report breaks down which counties have the biggest overprescribers of opioids. That information can help local authorities with how to curb overprescribing, she said.

States have been trying to get a handle on the opioid epidemic. Several states, such as Arizona, Maryland, and Florida, have declared a state of emergency over the opioid epidemic.

Federal agencies are trying to find ways to improve training.

The Food and Drug Administration recently added immediate-release opioids to a safety requirement for long-acting opioids that includes funding additional training for prescribers. And after a FDA request, Endo International pulled its painkiller Opana ER off the market, the first opioid that the FDA has asked to be removed from the market because of abuse.